The present invention relates to physical vapor deposition of titanium nitride.
Titanium nitride has been used as a barrier and adhesion layer in fabrication of tungsten plugs in semiconductor integrated circuits. Tungsten plugs interconnect different conductive layers separated by a dielectric. Frequently used dielectrics are silicon dioxide and silicon nitride. Tungsten does not adhere well to silicon dioxide and silicon nitride, so titanium nitride has been used to promote adhesion. In addition, titanium nitride serves as a barrier layer preventing a chemical reaction between WF6 (a compound from which the tungsten is deposited in a chemical vapor deposition process) and other materials present during tungsten deposition. See xe2x80x9cHandbook of Semiconductor Manufacturing Technologyxe2x80x9d (2000), edited by Y. Nichi et al., pages 344-345.
FIGS. 1, 2 illustrate a typical fabrication process. A dielectric layer 110 is deposited over a layer 120 which can be a metal or silicon layer. A via 130 is etched in the dielectric. A thin titanium layer 140 is deposited over dielectric 110 and into the via 130 to improve contact resistance (the titanium dissolves the native oxide on layer 120). Then titanium nitride layer 150 is deposited. Then tungsten 160 is deposited by chemical vapor deposition (CVD) from tungsten hexafluoride (WF6). Tungsten 160 fills the via. Layers 160, 150, 140 are removed from the top surface of dielectric 110 (by chemical mechanical polishing or some other process). See FIG. 2. The via remains filled, so the top surface of the structure is planar. Then a metal layer 210 is deposited. The layers 160, 150, 140 in via 130 provide an electrical contact between the layers 210 and 120.
Titanium nitride 150 can be deposited by a number of techniques, including sputtering and chemical vapor deposition (CVD). Sputtering is less complex and costly (see xe2x80x9cHandbook of Semiconductor Manufacturing Technologyxe2x80x9d, cited above, page 411), but the titanium nitride layers deposited by sputtering have a more pronounced columnar grain structure. FIG. 3 illustrates columnar monocrystalline grains 150G in titanium nitride layer 150. During deposition of tungsten 160, the WF6 molecules can diffuse between the TiN grains and react with titanium 140. This reaction produces titanium fluoride TiF3. TiF3 expands and causes failure of the TiN layer. The cracked TiN leads to a higher exposure of TiF3 to WF6, which in turn leads to the formation of volatile TiF4. TiF4 causes voids in the W film which are known as xe2x80x9cvolcanoesxe2x80x9d. To avoid the volcanoes, the sputtered titanium nitride layers have been made as thick as 40 nm, and at any rate no thinner than 30 nm. In addition, the sputtered titanium nitride layers have been annealed in nitrogen atmosphere to increase the size of the TiN grains.
The inventor has determined that under some conditions thinner annealed layers of sputtered titanium nitride unexpectedly provide better protection against the volcanoes than thicker layers. In some embodiments, fewer volcanoes have been observed with a TiN layer thickness of 20 nm than with 30 nm. In fact, no volcanoes have been observed in some structures formed with the 20 nm TiN layers. Why the thinner TiN layers provide better protection is not clear. Without limiting the invention to any particular theory, it is suggested that perhaps one reason is a lower stress in the thinner annealed layers and a higher density of the TiN grains.
The invention is applicable to physical vapor deposition techniques other than sputtering. Additional features and embodiments of the invention are described below.